The present invention relates to a superconducting magnet system which is for use in generating an intense magnetic field in various systems, such as a linear motorcar, a beam accelerator, and in the measurement of the magnetized material characteristics.
In a conventional superconducting magnet system of the type described, coils of magnetic systems were put into a superconducting state by immersing the coil in liquid helium to cool the coils to an extremely low temperature.
However, use of liquid helium renders a running cost high and handling difficult in the conventional superconducting magnet system. This is because the liquid helium is expensive, volatile, and difficult in handling. Further, the conventional superconducting magnet system inevitably becomes bulky in structure, since it needs a liquid helium tank, a liquid helium transfer tube, and the like.
Recently, in order to solve such disadvantages of the conventional superconducting magnet system, a superconducting magnet system which may be free from liquid helium has been proposed by the inventors of the present invention in Japanese Patent Prepublication No. 258103/1992.
The superconducting magnet system mentioned in the above-referenced application comprises a cryocooler which has a cooling stage, a superconducting coil which contacts with the cooling stage, and current leads for supplying an electric current to the superconducting coil. The cooling stage is kept at a predetermined cooling temperature. The superconducting coil is cooled down to the predetermined cooling temperature by the cryocooler. The cryocooler may have an additional cooling stage.
In the superconducting magnet system described in the above-mentioned application, no consideration has been made about the current leads which are used for supplying the electric current to the superconducting coil. In this connection, such current leads are formed by a normal conductive material.
However, when the current leads are formed by a normal conductive material, it has been found out that Joule's heat inevitably generates from the current leads during supply of the electric current to the superconducting coil. The Joule's heat is propagated into the superconducting coil and deteriorates an efficiency of cooling. As a result, a heavy load is imposed on the cryocooler on cooling the superconducting coil.
In order to solve the above-mentioned problem, the current leads may be formed by a high-temperature superconducting material. According to this structure, Joule's heat might not generate from the current leads while the current leads are kept at a superconducting state. However, it becomes necessary to cool the current leads by another cryocooler which is exclusively used therefor. Consequently, the superconducting magnet system inevitably becomes bulky in size and complicated in structure.
The superconducting magnet system mentioned in the above-referenced application has the other disadvantages.
Namely, it takes a long time to cool the superconducting coil from a room temperature to the above-mentioned superconducting state at an extremely low temperature lower than about 77K. In addition, the distribution of the temperature is not uniform in the superconducting coil in the superconducting state.